1. Field of the Invention
The present invention relates to a precompression pump with a feature to evacuate air trapped in the pump chamber to thereby assist in priming the pump. In particular, the present invention relates to a precompression pump which uses mechanisms to create gaps of very small size to bridge two seals on a pump piston, in a way that the small size of the gaps allows the passage of trapped air, but because of the viscosity of any liquid in the pump chamber does not allow the passage of liquid past the seals on the pump piston.
2. Description of the Prior Art
A number of different patents describe precompression pumps with mechanisms for evacuating air from the pump chamber to assist in priming of the pump. For example, U.S. Pat. Nos. 3,746,260; 3,774,849; 4,051,983; 4,144,987; 4,317,531; 4,365,729; 4,437,588; 4,530,449; 5,064,105 and European Patent No. 0 346 167 all show mechanical mechanisms in the form of projections or grooves, which create a passage around a single seal of a pump piston or valve seal to thereby create a passage for fluid to exit the pump chamber.
Difficulties have been encountered in precompression pumps using priming features of the prior art. For example, priming features which exhaust the air and/or liquid in the pump chamber past the inlet valve, as in, e.g., U.S. Pat. Nos. 4,051,983; 4,144,987; and 4,437,588. can result in air remaining in the dip tube below the inlet valve. A result of this arrangement is that undesirable variations in dosage size can result because the pump chamber does not fill with liquid but instead with a liquid-air mixture. Furthermore, priming features which exhaust the air and/or liquid in the pump chamber past the pump piston, as in, e.g., U.S. Pat. Nos. 3,774,849; 4,317,531; 4,3365,729; 4,530,449; 5,064,105; and E.P.O. Patent No. 0 346 167, often will leave residue in the area above the pump piston lower seal, which residue can clog or impede pump performance. In addition, these priming arrangements around the pump piston often require that an exhaust port be provided through the pump chamber sidewall between the top and bottom pump piston seals. This exhaust port can be difficult to mold and can impede the range of motion or designs available for the pump piston.
The prior art priming arrangement are generally of a relatively large size in comparison to the size of the piston and cylinder diameters, such that a single groove or projection provides a passage of a sufficiently large area for exhausting compressed air from the pump chamber. Such large priming arrangements can cause undesirably large deflections of the seal being bridged, resulting in scoring or fatigue in the portion of the seal immediately adjacent the projection or groove. As a result, the pump can have a reduced lifetime or reduced effectiveness over a long lifetime.